Dynamics and Geometry of Extratropical Cyclones in the Upper Troposphere by a Neighbor Enclosed Area Tracking Algorithm

Abstract

his study shows that the morphological characteristics of upper-tropospheric extratropical eddies are closely related to the background flow in the Northern Hemisphere winter. Enclosed surfaces of 300-hPa relative vorticity are identified by using the neighbor enclosed area tracking algorithm, and the periphery of these surfaces are approximated by ellipses. Eddies are classified into five categories according to the approximate ellipse. Eddies having an oblateness of less than 0.6 are classified as near circle, or are otherwise classified as northeast?southwest (NE?SW), northwest?southeast (NW?SE), north?south, or west?east, according to the direction of the major axis. In the wintertime climatology, NE?SW-oriented cyclones are collocated with the jet stream, while NW?SE-oriented cyclones mostly reside north of the jet. In interannual variability, moreover, the frequency of NE?SW cyclones is slightly correlated with the Arctic Oscillation (AO) index, while the frequency of NW?SE cyclones is highly anticorrelated with the AO index. This is consistent with positive feedback between horizontally slanted eddies and background flow, as has been shown in many previous studies.